A choke valve is a throttling device commonly used as part of an oil or gas field wellhead to reduce the pressure of the fluid flowing through the valve. A choke valve is placed on the production “tree” of an oil or gas wellhead assembly to control the flow of produced fluid from a reservoir into the production flow line, and is used on wellheads located on land and offshore, as well as on wellheads located beneath the surface of the ocean (sub-sea). Examples of choke valves used in oil and gas fields are shown in U.S. Pat. No. 4,540,022, issued Sep. 10, 1985, to Cove and U.S. Pat. No. 5,431,188, issued Jul. 11, 1995 to Cove. Both patents are commonly owned by the applicant of this application, Master Flo Valve, Inc.
In general, choke valves include:
a valve body having an axial bore, a body inlet (also termed inlet bore) which is typically oriented as a side outlet to the axial bore, and a body outlet (also termed end outlet or outlet bore) which is aligned with the axial bore;
a “flow trim” mounted in the bore between the inlet and outlet, for throttling the fluid flow moving through the body; and
biasing members such as a stem and bonnet assembly for actuating the flow trim to open and close the choke valve, and for closing the upper end of the axial bore remote from the outlet.
There are four main types of flow trim commonly used in commercial chokes, each of which includes a port-defining member forming one or more flow ports, a movable flow control member for throttling the flow ports, and seals to implement total shut-off. These four types of flow trim can be characterized as follows:
(1) a needle and seat flow trim comprising a tapered annular seat fixed in the valve body and a movable tapered internal plug for throttling and sealing in conjunction with the seat surface;
(2) a multiple-port disc flow trim, having a fixed ported disc mounted in the valve body and a rotatable ported disc, contiguous therewith, that can be turned to cause the two sets of ports to move into or out of register, for throttling and shut-off;
(3) a cage with internal plug flow trim, including a tubular, stationary cylindrical cage, fixed in the valve body and having ports in its side wall, and an internal plug movable axially through the bore of the cage to open or close the ports. Shut-off is generally accomplished with a taper on the leading edge of the plug, which seats on a taper carried by the cage or body downstream of the ports; and
(4) a cage with external sleeve flow trim, including a tubular stationary cylindrical cage having ports in its side wall and a hollow cylindrical external sleeve (also termed external flow collar) that slides axially over the cage to open and close the ports. The shut-off is accomplished with the leading edge of the sleeve contacting an annular seat carried by the valve body or cage.
In each of the above, the flow trim is positioned within the choke valve at the intersection of the choke valve's inlet and outlet. In the latter two types of valves, termed “cage valves”, the flow trim includes the tubular, stationary cylinder referred to as a “cage”, positioned transverse to the inlet and having its bore axially aligned with the outlet. The cage has one or more restrictive flow ports extending through its sidewall. For cage valves, flow through the ports of the cage is controlled by a flow control member which is either an internal plug component, or an external sleeve/flow collar component. Fluid enters the cage from the choke valve inlet, passes through the flow ports and changes direction to leave the cage bore through the valve outlet.
A problem that has produced many production interruptions both with surface and sub-sea facilities is the failure of valve trim due to fracture and cracking. The common cause of this fracture is foreign debris moving through the flow line into the valve with sufficient mass and velocity to damage or fracture the valve trim. High vibration and/or excessive side loads may also result in fracture of the flow trim. Fractures or cracking of the flow trim may be extreme, causing catastrophic failure of the choke valve, which results in over pressurization of the downstream equipment or damage to the well formation due to excessive flow.
Choke valve flow trim components are typically manufactured from hardened, high wear material such as a tungsten carbide material, while the valve body is formed of softer material, typically steel. The steel body is machined in the course of fabrication and must cope with stresses, and thus is manufactured from a relatively ductile steel. The flow trim however has harder surfaces. Typically the cage component of the flow trim is formed of tungsten carbide, and depending on the type of cage valve, the internal plug is formed of tungsten carbide, or a tungsten carbide liner is shrink-fitted as a liner in the flow collar. This is important because the flow trim is positioned at the bend of the “L”, where it is exposed to, and temporarily contains, the fluid flow when it is accelerated, is changing direction, and is in a turbulent state. Erosion of the flow trim may be extreme, causing catastrophic failure of the choke valve, which results in over pressurization of the downstream equipment or damage to the well formation due to excessive flow.
The tungsten carbide material in the flow trim is a powder metallurgy product where tungsten in the concentration of about 85-95% is bonded within a matrix with a binder material, typically nickel, cobalt, molybdenum, chromium or a combination of these elements in the concentration of about 5-15%. This produces a material that is very hard by nature to prevent or delay the effects of erosion to the valve trim. The hardness of the trim components is typically in the Rockwell Ra 90-95 range. As one increases the hardness of the tungsten carbide material (generally by decreasing binder materials in the matrix), the wear values are dramatically improved. However, as the hardness level of tungsten carbide is increased, the susceptibility to fracture also increases.
There have been a number of attempts at solutions to mitigate the damage caused by foreign debris; however, many of these attempts have resulted in a trade off to the potential wear capabilities of the choke trim. One approach is to manufacture the flow trim from tungsten carbide grades that have a higher percentage of binder material, typically nickel, cobalt, molybdenum, chromium or a combination of these elements. Concentrations in the order of 12-15% have resulted in improved toughness of the tungsten carbide matrix; however, the volume of binder in the concentration results in a matrix with lower hardness and consequently substandard erosion characteristics. This results in rapid wear to the flow trim resulting in costly production interruptions for valve maintenance or in the sub-sea applications, a high valve retrieval cost to facilitate maintenance.
Another approach is to encase the tungsten carbide material with a stainless steel carrier to absorb some of the energy from the foreign particle on impact and to protect the brittle tungsten carbide from direct impact. This approach has been used for the cage with internal plug flow trim, for example as shown in the cage component of U.S. Patent Publication 2010/0288389 A1, to Hopper et al., and assigned to Cameron International Corporation. The cage itself is a stainless steel carrier into which an internal tungsten carbide insert is press fit. The plug may also be formed from tungsten carbide. This design works well for the cage/plug flow trim, where the wear is to the internal bore of the cage. However, the internal plug design has proven inferior for erosion resistance when compared to the external sleeve (flow collar) choke valve designs. In the latter type of choke valves, the wear is extreme at the port areas between the external sleeve and the internal cage. For this reason a hard material such as tungsten carbide must be used at the interface between the cage and the external sleeve. Using a steel sleeve over a tungsten carbide insert, as is done with the cage with internal plug design, would result in accelerated erosion for cage valves of the external sleeve choke valve design.
Examples of such choke valves of the external sleeve cage valve design are shown in, for instance, U.S. Pat. No. 4,540,022, issued Sep. 10, 1985, to Cove et al., and U.S. Pat. No. 6,105,614, issued Aug. 22, 2000 to Bohaychuk et al. A choke valve including an external sleeve flow trim in sub-sea applications is shown in U.S. Pat. No. 6,782,949 to Cove et al. These patents describe the beneficial characteristics of the external sleeve (flow collar) design in erosion control, valve outlet erosion protection, seating integrity, and fluid energy control features. U.S. Pat. No. 8,490,652 to Bohaychuk et al., issued Jul. 23, 2013, discloses a cage component formed with tubular inner and outer cage members bonded together at an interface such as by brazing. This cage component can be used to reduce fracturing as the outer cage member may be formed from grade of tungsten carbide for fracture resistance, while the inner cage component may be formed from a harder, erosion resistant grade of tungsten carbide.
U.S. Pat. No. 7,426,938, issued Sep. 23, 2008 to Bohaychuk et al. relates to a choke valve with external sleeve and cage flow trim designed for fracture prevention. The valve includes a protective tubular sleeve, or insert cartridge in which the side ports are located to overlap with the intersection of axes of the inlet and outlet bores in a manner to avoid direct impingement of fluid along the axis of the inlet bore. In the cage, at least a pair of main flow ports are located to overlap with the intersection of the axes of the inlet and outlet bores, and are aligned with the side ports of the tubular sleeve or cartridge to communicate directly with the side ports. In this manner fluid enters the choke valve through the inlet bore and passes through the main flow ports at reduced pressure and continues out through the outlet bore, without direct impingement on the side wall of the flow trim components.
FIG. 1 shows a typical prior art choke valve in which the flow trim includes an external tubular throttling sleeve (flow collar) that slides over the cage. The sleeve acts to reduce or increase the area of the flow ports. An actuator, such as a threaded stem assembly, is provided to bias the sleeve back and forth along the cage. The rate that fluid passes through the flow trim is dependent on the relative position of the sleeve on the cage and the amount of port area that is revealed by the sleeve.
In sub-sea wellheads, maintenance cannot be performed manually. An unmanned, remotely operated vehicle, referred to as an “ROV”, is used to approach the wellhead and carry out maintenance functions. To aid in servicing sub-sea choke valves, such choke valves have their internal components, including the flow trim, assembled into a modular sub-assembly. The sub-assembly is referred to as an “insert assembly” and is inserted into the choke valve body and clamped into position. FIG. 2 shows a typical prior art sub-sea choke valve with flow trim of the external throttling sleeve (flow collar) type.